AFAIK the site itself, and mods (unknown who they are besides pg really) kill posts at their own whim. Flagging is one signal for that. Of course, you can just wait for the passage of time, which will "kill" any post on the front page eventually.
That's what I meant by "the site iteslf", sorry I wasn't clear. There's definitely some automated spam-detection, and voting circle detection, which auto-bans some posts.
Posts don't need upvotes to live, and don't need downvotes to die. I imagine there's some ranking done based on the number of upvotes or comments in the last X period against the amount of time it's been up at all.
